Real Translucency (NOT Transparency or SSS) in Principled Material Setup
I'm not sure why there is no Translucency option on the Principled shader. Perhaps there is a way to get translucent behavior from the Principled shader, but not that I know of. Here's a possible way to combine the two shaders, which should take thickness into account.
There's an answer here that goes over a way to find the approximate thickness of an object and use it in a shader by using the Ambient Occlusion node with Inside enabled.
While I'm not sure of your exact situation, I think it could be a reasonable way to mix the main shader with the translucency. I'd start by seeing what the Ambient Occlusion node outputs. It might be good enough as is. You'll probably need to adjust the distance though. I'd set it a bit higher than the thickest point on your mesh that you want to be translucent.
In some quick tests I did, I found the output might not be perfect on it's own. However, it's a pretty good start. You may want to mix in the Pointiness output of the Geometry node to isolate any raised sections (bumps, veins, etc.) that shouldn't be translucent. Mixing the two outputs using a Math node set to Maximum seemed to work well.
A quick hand painted texture could be used to &quot;mask&quot; the general areas that you want translucent. Add a Math node set to Multiply to add in the texture. This would just ensure that nothing is translucent that shouldn't be.
Here's a really quick test I did as a proof of concept. Far from perfect, but I think the idea is there. The dark spot is a Suzanne that I placed just underneath as a test. The mesh is just a subdivided cube with a thin section in the center and a couple sculpted &quot;veins&quot;.

And the node setup:

As you can see there is a few spots on the outer edge that ended up translucent. With some tweaking of the ColorRamps it might get a bit better, but manually masking might be the way to go.